It's difficult to recommend a single color scheme since people can have different preferences. Some like darker backgrounds while others like lighter ones. I searched the internet for CAD programs to see what viewport colors they use and from what I found and given that FreeCAD has a lighter UI color, I think something like this could work:

Nice proposals. Colors used look balanced. And i guess it shows a designer put them together.

Even better would be that, there can be a 'Presets' drop-down menu in the preferences that includes a few color schemes by default which users can select from or add their own.

As there already is an option available for customization. I don't feel presets are absolutely necessary. At some point in the future FreeCAD could i guess evolve default color scheme as seen in the viewport. Something unique for FreeCAD and for end users to accept it in general. Likely we are far from that to happen. But i guess your proposals are a nice first step.

It's difficult to recommend a single color scheme since people can have different preferences. Some like darker backgrounds while others like lighter ones. I searched the internet for CAD programs to see what viewport colors they use and from what I found and given that FreeCAD has a lighter UI color, I think something like this could work:

Maybe creating different themes that those interested might toggle on or off. In my opinion the current freecad stands out in it's appearance and general feel with it's default background color. but increasingly for those who spend countless hours designing on computer screens might love a default color that is not too hard to their eyes. the bright colors can strain the eye at some point for heavy users. meanwhile you don't want a completely darklike autocad etc, so a good medium is great.

@regis
I agree, having presets for background colour would be convenient. The default FreeCAD background does well as a dark theme. In some cases, users that spend long hours in front of the computer might prefer a more neutral dark colour.